MELBOURNE, May 10 - Carmen Lahiff-Jenkins said her son Quinn, 14, was set upon by five youths after they challenged him to meet them outside a Northcote school, where he was pushed from his bike and attacked with spanners.

"No adult stopped to help, no one beeped their horn or did anything from their cars, which of course is another thing which is very distressing," Ms Lahiff-Jenkins told 3AW radio on Thursday.

She said two of the five unknown youths were armed with spanners.

"It's my husband's and my worst nightmare, and I know its the worst nightmare of most parents who have an autistic child," she said.

Lahiff-Jenkins said her son had messaged the teens on social media and accused them of bullying his friend, before the incident escalated.

"Quinn was flipped off his bike and beaten with spanners, kicked, punched and hounded around Northcote tramstop," his mother said.

"I was aware when he contacted me from within the ambulance and said, 'Help, Mum, this is what happened to me'."

Premier Daniel Andrews described it as a "terrible incident".

"As I understand it the school has taken this matter very seriously, as they should," he said.

"As bad as this incident was, as difficult as it is to comprehend, it's one of a very small number of incidents that we see across our schools."

Chief Commissioner Graham Ashton said police were investigating.

"I watched that myself and [it's] really just disgusting behaviour against this young fella," he told ABC Radio.

Meanwhile, in a separate case, Ashton said police were "confident" in the actions of two Protective Services Officers filmed pinning down an injured teenager at a Melbourne train station.

The teenager, accused of pushing a PSO, broke his ankle trying to flee down some stairs.

The 16-year-old was arrested at Bayswater train station on Sunday on suspicion of criminal damage and after initially providing a fake name.